Liquid crystals have found use in a variety of electro-optical and display device applications, in particular those which require compact, energy-efficient, voltage-controlled light valves such as watch and calculator displays. Liquid crystal displays have a number of unique useful characteristics, including low voltage and low power of operation. In such displays, a thin layer of liquid crystal material is placed between glass plates and the optical properties of small domains in the layer is controlled by the application of electric fields with high spatial resolution. These devices are based upon the dielectric alignment effects in nematic, cholesteric and smectic phases of the liquid crystal compound in which, by virtue of dielectric anisotropy, the average molecular long axis of the compound takes up a preferred orientation in an applied electric field. However, since the coupling to an applied electric field by this mechanism is rather weak, the electro-optical response time of liquid crystal based displays may be too slow for many potential applications such as in flat-panel displays for use in video terminals, oscilloscopes, radar and television screens. Fast optical response times become increasingly important for applications to larger area display devices. Insufficient nonlinearity of liquid crystal based displays can also impose limitations for many potential applications.
Electro-optic effects with sub-microsecond switching speeds can be achieved using the technology of ferroelectric liquid crystals (FLCs) of N. A. Clark and S. T. Lagerwalll (1980) Appl. Phys. Lett. 36:899 and U.S. Pat. No. 4,367,924. These investigators have reported display structures prepared using FLC materials having not only high speed response (about 1,000 times faster than currently used twisted nematic devices), but which also exhibit bistable, threshold sensitive switching. Such properties make FLC based devices excellent candidates for light modulation devices including matrix addressed light valves containing a large number of elements for passive displays of graphic and pictorial information, optical processing applications, as well as for high information content dichroic displays.
Smectic C liquid crystal phases composed of chiral, nonracemic molecules possess a spontaneous ferroelectric polarization, or macroscopic dipole moment, deriving from a dissymmetry in the orientation of molecular dipoles in the liquid crystal phases (Meyer et al. (1975) J. Phys. (Les Ulis, Fr) 36:L-69). The ferroelectric polarization density is an intrinsic property of the material making up the phase and has a magnitude and sign for a given material under a given set of conditions. In ferroelectric liquid crystal display devices, like those of Clark and Lagerwall, appropriate application of an external electric field results in alignment of the chiral molecules in the ferroelectric liquid crystal phase with the applied field. When the sign of the applied field is reversed, realignment or switching of the FLC molecules occurs. This switching can be employed for light modulation. Within a large range of electric field strengths, the switching speed (optical rise time) is inversely proportional to applied field strength and polarization or dipole density (P), and directly proportional to orientational viscosity. Fast switching speeds are then associated with FLC phases which possess high polarization density and low orientational viscosity.
A basic requirement for application of ferroelectric liquid crystals in such devices is the availability of chemically stable liquid crystal materials which exhibit ferroelectric phases (chiral smectic C.) over a substantial temperature range about room temperature. Useful device operating temperatures range from about 10.degree. C. to about 80.degree. C. More typical device operating temperatures range from 10.degree. C. to 30.degree. C. In some cases, the ferroelectric liquid crystal compound itself will possess an enantiotropic or monotropic ferroelectric (chiral smectic C* ) liquid crystal phase. Ferroelectric liquid crystal mixtures possessing chiral smectic C* phases with useful temperature ranges can also be obtained by admixture of chiral, nonracemic compounds, designated ferroelectric liquid crystal dopants, into a liquid crystal host material (which may or may not be composed of chiral molecules). Addition of the dopant can affect the ferroelectric polarization density and/or the viscosity of the C* phase and thereby affect the switching speed. Desirable FLC dopants are molecules which impart high ferroelectric polarization density to an FLC material without significantly increasing the orientational viscosity of the mixture. The components of FLC mixtures can also be adjusted to vary phase transition temperatures or to introduce desired LC phases.
In addition to the above-described characteristics, the composition of ferroelectric liquid crystal materials can be adjusted to vary the tilt angle, pitch, stability and mixing properties of the FLC materials. Addition of molecules which optimally impart a 22.5.degree. tilt angle to an FLC material used in a shutter or light switch, results in maximum throughout in the "ON" state of the device. A 22.5.degree. tilt angle is particularly desirable for FLC materials used in direct drive, flat panel display applications. A longer helix pitch in the smectic C* phase, particularly a pitch longer than about 3.0 .mu.m, is also a desirable characteristic of FLC materials for certain applications, since such a longer helix pitch improves the alignment of the FLC compounds in electro-optical devices, decreases surface interactions and as a consequence improves the usefulness of these compounds in SSFLC (Surface Stabilized Ferroelectric Liquid Crystal) devices. FLC components can also be added which increase the stability of the smectic phases of the FLC material, for example, by suppressing crystallization of FLC materials, and/or improving the miscibility and/or viscosity of the liquid crystal composition.
Thermotropic liquid crystal molecules typically possess structures which combine a rigid core coupled with two relatively "floppy" tails (see Demus et al. (1974) Flussige Kristalle In Tabellen, VEB Deutscher Verlag fur Grundstoffindustrie, Lebzig for a compilation of the molecular structures of LC molecules). FLC materials have been prepared by the introduction of a stereocenter into one of the tails, thus introducing chirality. The first FLC compound to be characterized was DOBAMBC (Meyer et al., supra) which contains an (S)-2-methylbutyloxy chiral tail. Pure DOBAMBC exhibits a smectic C* phase with a ferroelectric polarization of -3 nC/cm.sup.2.
There are a number of reports of compounds containing two or more aromatic rings such as those having phenylbenzoate, biphenyl, phenylpyrimidine, phenylpyridine and related cores coupled to chiral tail units which possess smectic C* phases displaying fast switching speeds at room temperature, or which can be employed as FLC dopants to induce high polarization and fast switching speeds when combined in mixtures with FLC host materials. There are also several reports of FLC compounds having cores which contain cyclohexane and cyclohexene rings.
The following are exemplary reports of FLC compounds containing cyclohexane or cyclohexene rings:
Li et al. (1991) Mol. Cryst. Liq. Cryst. 199:379-386 disclose cyclohexenyl liquid crystal compounds, having a chiral center in the mesogenic core, derived from the Diels-Alder reaction between myrcene and methyl acrylate, followed by hydrolysis and esterification with4-hydroxy-4'-n-alkoxybiphenyl. The liquid crystal materials reported have the following structure: ##STR2## The lower member cyclohexenecarboxylates (n =1 and 2) have a large nematic range, and the higher members (n=3-10) have multiple smectic phases in addition to the nematic phase. The presence of the cyclohexene ring is suggested to lead to multiple smectic phases. Nonracemic 4'-n-octyloxybiphenyl 4-(4-methyl-3-pentenyl)-3-cyclohexenecarboxylate is reported to have a smectic C phase and a normal tilt angle, however, its polarization density is extremely small (extrapolated polarization density, P.sub.ext, less than 1 nC/cm.sup.2). It is suggested that the small polarization density is due to the small dipole associated with the chiral carbon in the tilt plane, which does not contribute to P. Further, the carbonyl adjacent to the chiral produces nearly equivalent, but opposite, dipole moments in the two potential configurations, which occur with nearly equal probability.
Fung et al. (1989) Mol. Cryst. Liq. Cryst. Let. 6(6):191-196 report liquid crystal compounds containing a cyclohexene ring, derived from the Diels-Alder reaction between mycrene and methyl acrylate, followed by hydrolysis and esterification of the resulting acid with 4-hydroxy-4'-methoxybiphenyl or 4-hydroxy-4'-cyanobiphenyl. The liquid crystal materials reported have the following structure: ##STR3## where X is a methoxy or cyano group. The two compounds exhibit broad nematic ranges (79-153.degree. C. and 93-152.degree. C., respectively).
Bezborodov et al. (1989) Liq. Cryst. 4(2):209-215 disclose the mesomorphic (nematic) properties of cyclohexenyl liquid crystal compounds derived from 4-substituted phenols and 4-n-alkylcyclohexene-1-carbonyl chlorides where the double bond is in the 1, 2 or 3 position in the cyclohexene ring. The reference indicates that compounds containing the double bond in the 2 position of the cyclohexene (numbering from the carboxy group as is conventional) are the most promising for use as liquid crystal components, since the appearance of the double bond in the first or 3 positions of the ring causes a large distortion in the shape of the molecule. This distortion reportedly affects both the mesophase (nematic) range and the melting point.
German patent document, Reiffenrath et al., DE 3906040, published Sep. 21, 1989 and WPI Abstract 89-279241/39, refers to cyclohexene derivatives having the general formula: EQU R.sub.1 --A.sub.1 --Z.sub.1 --A.sub.2 --(Z.sub.2 --A.sub.3)m--R.sub.2
where R.sub.1 and R.sub.2 are 1-15 carbon alkyl or 3-15 carbon alkenyl groups, optionally with one CN or at least one flourine or chlorine substituent, in which a CH.sub.2 group can be replaced with --O--, --OCO--, --COO-- or --OCOO--, and one of R.sub.1 and R.sub.2 can be CN; where A.sub.1, A.sub.2 and A.sub.3 can be 1,4-cyclohexenylene or trans-1,4-cyclohexylene in which one or two non-adjacent CH.sub.2 groups can be replaced by --O--, or 1,4-phenylene, optionally with one or two fluorine substituents, in which one or two CH.sub.2 groups can be replaced by nitrogen, at least one of A.sub.1-3 being 2,3-difluoro-1,4-phenylene, and at least one of A.sub.1-3 being 1,4-cyclohexenylene; and where Z.sub.1 and Z.sub.2 can be --COO--, --OCO--, --CH.sub.2 O--, --OCH.sub.2 --, --CH.sub.2 CH.sub.2 -- or a single bond. The reference refers to 1,4-cyclohexenylenes having the double bond in the 1, 2 or 3 position.
Tanaka et al. (1989) European Patent Application, Pub. No. 331091 refers to tetracyclic cyclohexylcyclohexene derivatives having the formula: ##STR4## where R is a straight-chained alkyl group having 1-9 carbon atoms; A is a cyclohexyl, cyclohexenyl, or phenyl ring; B and C are cyclohexyl or clyclohexenyl rings; n is 0 or 1; when n is 0, X is a cyano group and Y is a hydrogen or fluorine atom; when n is 1, X is a fluorine atom, a straight-chained alkyl group having 1-9 carbon atoms, and Y is a hydrogen or fluorine atom. The disclosed liquid crystal compounds exhibit high N-I and low C-N or S-N points.
Eidenschink et al., WO 87/05015, discloses cyclohexane containing liquid crystal and ferroelectric liquid crystal compositions having the general formulas: EQU R.sup.1 --A.sup.1 --Z.sup.1 --A.sup.2 --R.sup.2
where A.sub.1 and A.sub.2 can be a phenyl, cyclohexyl, phenylpyrimidine, or substituted cyclohexene ring. Eidenschink et al. does not specifically disclose a cyclohexene ring, but generically discusses reduced groups at page 14, fourth paragraph, and suggests that the claimed compounds can include reduced groups.
While a number of useful liquid crystal and smectic liquid crystal materials (both pure compounds and mixtures) have been reported, there is a growing need for LC and FLC materials with varying properties of pitch and tilt angle for use in varied applications. In order to obtain faster switching speeds, FLC materials with low orientational viscosity are desirable. Further, there is a need for LC host materials and FLC dopants with varying mixing properties (which are dependent, at least in part, on chemical composition) for use in the preparation of FLC mixtures having desired chiral smectic phases at useful device operating temperatures (e.g. about 0.degree.-100.degree. C., preferably around room temperature about 10.degree.-35.degree. C). LC and FLC materials which result in mixtures that are stable to crystallization over useful device operating temperatures are desirable. LC host materials and FLC dopants which are readily synthesized and which impart longer chiral smectic phase pitch, tilt angle of about 22.5.degree., lower orientational viscosity, broader LC and FLC phases, and suppress crystallization in such mixtures are of particular interest.